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Why do passenger aircraft have the little airjets over each seat?
I never turn them on, but I do turn them off if I notice them. Is this a relic from the age when people lit up next to you and you wanted to blow away the smoke? The system must weigh almost a ton on a large aircraft and seems pretty unpopular and hard to maintain.

Having flown on older airliners without gaspers recently, they are indispensable. The cabin heats up a ton and it's a lot harder to maintain per-seat comfort without airflow.

Do you have a source for your "seems pretty unpopular" comment, or is this a personal opinion?  I for one welcome fresh air, and the ability to direct it and modulate it.

There's the psychological effect of giving the passenger something to fiddle with, that makes them feel a little bit in control of a situation where they would otherwise feel powerless.

The fact that you turn them off when you notice them is direct evidence that those air vents are popular with some people, otherwise you would find them turned off each time you found yourself sitting in an airplane.
In my case, I do use those little airjets, I'm overweight and my extra large forehead needs the extra cooling from time to time.

I love those air jets. I find few things more refreshing than the flow of air over my face when I'm trying to relax on an plane.

I feel it should be noted somewhere that those are by no means exclusive to airplanes. Overland buses built during the last two or three decades generally tend to have them, as well.

As an airline passenger I use these on nearly every flight. As a pilot in my own airplane, I use them even more and consider them to be (nearly) indispensable to the flight.

The airjet openings are connected to a long duct running along the ceiling. Since the air volume inside the cabin is restricted, fresh air must be pumped into it constantly. It is bleed air from the compressor section of the engine, which is first cooled and then enters the cabin via those ducts which feed all the little vents or airjets, as you call them. Passengers can open or close them individually and direct the flow for best comfort. The general idea is that some vents will be closed, but enough are open during every flight so the needed recirculation of air can happen. If more are closed, the pressure in the duct will be higher, so the flow through the remaining vents will increase.
The total weight of all the cabin ducting is a minor consideration, it's only a few dozen gram per passenger.

Cabin air recirculation will still happen if all the passenger vents are closed, through other vents in the ceiling

Why do airliners have them? Because passengers like them.
When you're sitting in the penalty box waiting for a departure slot and the air conditioning system is barely running the extra cool air you can get from those vents may be the only thing keeping passengers from strangling the screaming child kicking the seat.
Also a common "cure" for airsickness is fresh air -- blowing cool air over someone's face will often help with symptoms of motion sickness, and may prevent a nauseous passenger from actually vomiting. Opening these overhead "gasper vents" and directing them at the passenger's face provides a quick way of doing this.

+1 I always set it to low, and if possible, just in front of me (rather than on me). Since I'm not really feeling well during takeoff (getting better as I finish my 3rd dozen of flights), I truly appreciate that.

+1 what @yo' said. The fresh cabin air from the vents on most planes does tend to be rather cold and dry, which can make pointing the vent at full blast at your face kind of uncomfortable. Fortunately, though, the vents are adjustable, and opening yours just a little, and pointing it in front of you, saves you from having to breathe 300 other passengers' stale BO all the way.

The motion of an airplane doesn't bother me, but the air quality certainly does. Without the (slightly) better air from those vents, I'd just be miserable.

+1 for the airsickness: for those of us who suffer with motion sickness, the vent can be the difference between "An uncomfortable flight" and "Projectile vomiting". Trust me, if I'm throwing up on a plane, you'll want your own supply of fresh air.

So I realize this is by far and away the most popular answer, but when the answer to almost every other question that starts out with "why do" or "why don't" and contains the word "airlines", usually boils down to cost, I'm not so sure I agree with "because pax like them" as the reason they (still) exist.

@CGCampbell I'm pretty sure aircraft cleaning crews like them too - at least if they have a bunch of passengers on the plane with motion sickness - but I'm going to guess the incremental cost of the gasper vents versus fixed slits isn't significant, even in the cost of equipping a fleet, and if it's a differentiator in the otherwise cattle-like airline experience perhaps it's worth it to the airline. At least for now…

The air conditioning system on a plane can keep the cabin at a comfortable temperature no matter what the temperature outside is like, but it needs either the engines or the APU running (and the doors closed) in order to work correctly. Since these are often off while the plane is waiting at the gate, the air stream from the gaspers can make the passengers feel more comfortable until the engines are running.

The primary reason is that not everyone likes the same temperatures. I happen to be cold-natured and like to be warm. Other people are hot-natured and like to use copious amounts of air conditioning. These allow all of us to stay relatively comfortable in the same cabin.
Also, when you're sitting on the ramp in the summer, it's more than 100 degrees Farenheit outside, and the boarding door is still open, you'll learn to appreciate these things quickly, especially if you've been standing in the jetway a while before taking your seat. Personally, I usually turn mine off during flight, but I do tend to use them on the ramp and sometimes during taxi in the summer and/or unusually hot climates.

Lowest cost is king if you can get away with it.
Personal air vents provide more flexibility than if they were absent and the overall cost is lower.
If personal airjets are available then the jet temperature and maximum available flow can be set to suit people with preferences on the cold side of normal, while the mean temperature can be hotter than would suit a significant proportion of people.
If, instead, the cabin needed to be maintained at a temperature that most people would consider acceptable then, in the absence of personal vents, the whole cabin would need to be cooled to that temperature. Some people would then feel too cold, which can be dealt with by clothing or blankets, and most people would be comfortable.
In the first case you have lower than otherwise energy use with "outliers" being addressed with airjets.
In the second case you have higher than otherwise energy use with outliers being addressed with blankets. The energy cost of the blankets is zero but overall it still costs more this way.

FWIW I'm an air-jet loving traveller and greatly dislike aircraft without them.

Welcome to [aviation.se] and giving a very nice first answer here. An excellent slightly different view of the reasoning.

@CGCampbell Thanks. I'm an engineer (Electrical by qualification but anything technical by bent). This is the perspective the accountants come from and the engineers have to work out how to address :-). (Alas, it also means you end up with Lithium Ion batteries in DreamLiners, when Lithium Ferro Phosphate make way more engineering sense).

Although probably not the primary reason airlines have them in the first place, there is another great reason to use them.  Having fresh "outside" air blow in front of you displaces the viruses/bacteria/etc. from sick passenger's coughs and sneezes away from entering your nose and mouth.

The air coming out of the gaspers is exactly the same air that's blown into the cabin through the regular ventilation system.

Make the passenger feel he's in control of something.
The practical use is to fight rising temperatures and odors, but i also quite like the oxygen.


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What kind of wing-surface area would Curiosity need to be able to fly in the atmosphere of Mars?
The rover - without wings - has around 1000 kg of mass and Mars has 38% of Earths g. The density of the atmosphere is 1% of that of Earth. Assuming the surface area of the wings could be used for photoelectric cells having an efficiency of 40% at 500W/m^2, what wing dimensions, power and speed would this airborne rover need?

Related

There's a massive range for this, are we assuming a fixed amount of storable battery power? How long is the "runway" area available? How long should the flight be? (Time/distance?). What material is the wing made of? Are we to account for this in the weight of the lander? How much load must it handle? Should the wings be foldable for flight? This is more of a physics question, although for reference, a Cessna 172 is roughly 1000kg :)

Interplanetary Cessna: "X-Plane tells us that flight on Mars is difficult, but not impossible. NASA knows this, and has considered surveying Mars by airplane. The tricky thing is that with so little atmosphere, to get any lift, you have to go fast. You need to approach Mach 1 just to get off the ground, ..."

@MichaelKjörling have you clicked on my link? :P

@Federico No. Should I have? ;-)

@JonStory: Don't let XKCD mislead you: The Boeing Condor would be a much better basis for this task.

I actually hadn't seen the XKCD link when I commented - I was basing it on weight :)

I overlooked to close the iteration and had to revise the post. Sorry for all the confusion, but now the results make sense and the airplane is smaller.

Our Flight Environment
First the basics: Mars atmosphere is mainly carbon dioxide and has an average temperature of -47°C and an average pressure of 0.004 to 0.0087 bar. The speed of sound in these conditions is 238.2 m/s, the dynamic viscosity $\nu$ is 1.14$\cdot 10^{-5}$, and for my calculations I use an atmospheric density of 0.0234 kg/m³.
Now I owe all you early upvoters an apology. The earlier answer used an unsuitable methodology; now I have improved the answer with different results.
Operating Point
Next step: The electric power supply makes a propeller the right choice, and for propeller aircraft the minimum power to sustain flight can be achieved when the lift coefficient is
$$c_L = \sqrt{3\cdot c_{D0}\cdot\pi\cdot\Lambda\cdot\epsilon}$$
This makes your zero-lift drag the real sizing parameter: Make the aircraft a little more draggy, and you get punished with a big size increase. Therefore, we strive to keep the zero-lift drag coefficient $c_{D0}$ down to 0.008 in order to keep the aircraft small.
This allows to get an idea which aspect ratio will be best: At minimum power the total drag is four times the zero-lift drag, and the lift coefficient should be as high as possible to keep the aircraft's size small. Realistically, our flight $c_l$ will not grow above 1.6 (which is the best that low-speed airfoils can manage at a reasonable L/D), so the aspect ratio is
$$\Lambda = \frac{c_l^2}{3\cdot c_{D0}\cdot\pi\cdot\epsilon}$$
which yields an aspect ratio of 35.74 when $\epsilon$ is 0.95.
Available Power
Now we can follow the procedure in this answer to calculate power demands for your solar-electric propulsion. Solar radiation on Mars is a lot lower than on Earth, but very reliable as there are no clouds to obstruct the sun. Beware of dust storms, however!
Since you gave the Martian solar constant in your question, I will use it here: The wing will be covered to 80% with solar cells, and each square meter of them will produce your optimistic 200 W (500 W radiation times 40% efficiency). Per square meter of wing we can achieve 160 W, and with an electric efficiency of 96% and a propeller efficiency of 85% this translates into a propulsive power per square meter of 130.5 W.
Matching Thrust to Drag
Now we can equate the drag $D$ and the available thrust $T$ from the power $P$, like this:
$$D = T = \frac{P}{v} => 4\cdot c_{D0}\cdot S\cdot\frac{\rho\cdot v^3}{2} = 130.5\cdot S$$
Luckily, wing area $S$ can be canceled, and we get a result for the airspeed $v$ at which the aircraft's drag can just be supported by the power the wing can generate independent of wing area:
$$v = \sqrt[3]{\frac{130.5}{4\cdot c_{D0}\cdot\rho}} = 55.857 m/s$$
Now for the real size: If we assume that the payload fraction is 25%, the mass of the full aircraft will be 4 tons. To lift this at the dynamic pressure of 36.5 N/m² which corresponds to the found flight speed we need a wing area of 254 m². This can create 40.65 kW of power, 33.17 of which are transformed into thrust. The wing span will be a whopping 95.28 m. How you get this to Mars I leave to your imagination - after all, this is just a tad less than the wingspan of the Hughes H-4.
If you can achieve a higher payload fraction, the wing will become smaller: At 30% the full aircraft mass is only 3.333 tons, and the wing size will become 211.74 m² large. Its wing span will be 87 m. With these dimensions I doubt that a higher payload fraction will be possible. The closest aircraft to this design was the Boeing Condor, which had an empty mass of 3.6 tons (with a propulsion system weighing close to the mass of Curiosity) and a wing span of 61 m.
Last Checks

Mach number: 56 m/s is just Mach 0.2345 on Mars, so we should not run into compressibility effects which would limit the assumed lift coefficient.
Reynolds number. The bigger wing has an average chord of 2.66 m and the smaller one still 2.43 m chord, and per meter the Reynolds number in -47°C cold carbon dioxide at 56 m/s is 4,900,000 - plenty enough to make the selected lift coefficient possible. Size helps here!

Where does this value for the product of $c_l$ and $Ma^2$ come from? Why not use $L=c_l 0.5\rho V^2 S$?

@ROIMaison: I learned it from Mark Drela. It is better to express speed in terms of $Ma$, because the Mach number  limits your $c_{l max}$ at higher speeds. Just replace $v$ by $Ma \cdot a$, and your lift equation will contain all essential elements.

I understand, and will the value of 0.4 still be valid on Mars? As there is a different 'air'-density.

@ROIMaison: As long as the atmosphere behaves like an ideal gas the flow phenomena at the same Re and Ma numbers should be the same. And with its low pressure and temperature I have no doubt that the atmosphere on Mars is very similar to Earth's atmosphere 31 km up.

I'm afraid the available solar energy is only half - 500 W impinge on the surface, but solar cell efficiency is 40% at best, so it's 200 Watt/m^2. I should have multiplied the efficiency and the raw energy in my question!

@SebastianHenckel: I screwed up myself. I rewrote the answer using a different and hopefully better methodology, but I doubt that solar-powered aircraft will soon take off on Mars.

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How is the pressure difference in passenger plane toilets generated?
The toilets on passenger planes flush with an impressive amount of force - Is the negative pressure inside the tanks generated by a pump or are the tanks connected to the outside air pressure? Considering that the toilets should also work on the ground, I presume pumps are used - on the other hand, maybe the low pressure is stored inside the tanks from the last flight? Also, pumps would mean the toilets don't work when the aircraft is not powered up  - is this true? Also, how was the engineering problem solved of keeping the pumps or the tubes connecting to the outside free from waste?

How is the pressure difference in passenger plane toilets generated?

The pressure in flight is generated by using the vacuum outside. This dosen't work on the ground or at low altitude, so there's a pump as well to draw out the air to create an artificial vacuum.

Considering that the toilets should also work on the ground, I presume
  pumps are used - on the other hand, maybe the low pressure is stored
  inside the tanks from the last flight?
  Also, pumps would mean the toilets don't work when the aircraft is not
  powered up - is this true?

Not sure about this one, but since the aircraft is seldom not powered when a person is onboard (maintenance can't do much without power either), I can't think of this being a significant problem. There might be a little bit of leftover pressure.
Airbus states that the vacuum pump is powered off the AC power bus, so it may be powered from the ground or the APU when the engines aren't on.

Also, how was the engineering problem
  solved of keeping the pumps or the tubes connecting to the outside
  free from waste?

As is seen in the picture below, the tank is rinsed out through with the help of a second port on the access cover to clean it out. The rest of the system I believe is designed to last until the next maintenance, when (I think) it's connected up to a high pressure hose to thoroughly clean it out.

A320 Toliet System:

A320 System Details.

You may wish to edit the first line, as the vacuum is caused by the lower external pressure which is technically low vacuum but that may confuse some, not aware that vacuum aren't just breathing and hard vacuum of space. Just a thought

In this video is shown a test of the A380 system. At about the 0:48 mark the presenter remarks that

The toilets work by pumping air out of waste-tanks in the rear of the plane

So, at least in the A380 case, yes the negative pressure is achieved via pumps.
As for your last question

how was the engineering problem solved of keeping the pumps or the tubes connecting to the outside free from waste?

If you look at the video, you see that the pipes are connected on the top of the tank and the gravity will keep the waste away from the pump (in the assumption that no acrobatic maneuver will be performed, usually valid, but not always)

There is a vacuum-pump that draws the air out of the waste tanks. The whole system is under low pressure.
Here is a video explaining how the A380 toilet system works.

sorry, ninja'ed you on that video :D

@Federico Yeah, you were seconds faster :-). And Manfred came in within the same minute as well.

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Why are airplanes riveted and not screwed?
Replacing airframe panels and inspecting hard-to-reach parts would be easier with screws, I presume. So why are rivets used?

I read somewhere that there are plans for the planes to be glued. I do not have the source anymore but it makes sense: the stress is distributes, no weak points etc.

@WoJ Some aircraft manufacturers are now using Friction Stir Welding techniques, but I haven't heard of glue being used.

Airplanes are riveted, not screwed because they are the product of engineers, not lawyers.

@Lnafziger glue is very much on the table for fibre reinforced plastic structures. Only it is mostly termed adhesive and way more expensive.

@yankeekilo Are you referring to resin for composite aircraft?

@Lnafziger not so much to the matrix of FRP parts themselves but dedicated adhesives that are used to bond/glue different (cured) FRP components.

Fokker Aircraft used to bond (glue) many bits of the aircraft. A most reliable way of aircraft construction since you don't introduce stress concentration around the holes.

Rivets need to press the two parts together AND to transmit shear. The pressure results in friction which is responsible for a considerable part of the load transmitted between two riveted panels. In order to hold up to the maximum possible amount of shear, the rivet needs a smooth surface. A thread would make it much more vulnerable due to the notch effect of the thread.
Since skin panels and flanges vary in thickness, a LOT of different screw sizes would need to be kept on stock so that every combination of panel thicknesses is covered with a fitting screw which has just the right amount of smooth shank length. Rivets, on the other hand, will be hammered to the right length during manufacture, so only a few sizes are needed to cover all those panel thicknesses.
Historically, metalworking tools were much less precise than today. For optimum shear transmission, the bolt must sit tightly in its hole. In the past, this could  not be done with screws, but only with rivets which grow thicker when set in place. And since aviation regulation authorities are VERY conservative, the easiest to certify joints are those proven rivet joints.
Repairability is adequate when using rivets, especially if you can reach only one side. The rivets are drilled out with a drill bit which has a diameter slightly smaller than the original hole size. Great care is taken not to oversize their original hole. The rivets are then, in the case of sufficiently thick material, tapped out with a hammer and punch. If the material is too thin or fragile to allow a punch and hammer to be used, the rivet will be drilled out with a drill bit that matches the original size of the rivet hole, with great care taken not to oversize the hole. If the hole is damaged or oversized in the removal process, a larger hole may be drilled and corresponding rivet installed, depending on the conditions present. After inspection or repair the parts are riveted together again.
If you need the lowest mass for a given strength, again rivets win over screws. The difference might be small for the individual rivet, but it adds up for the whole airframe.

Bolts are heavier - like you said - and more expensive, particularly the high precision variety needed. Plus they are more difficult to mount: one technician inside the aircraft, one on the outside, twisting their wrenches until the prescribed preload torque is reached. Plus it is better to use many small diameter holes than few large diameter holes, and small bolts are just cumbersome.

However many aircraft make use of threaded permanent fasteners in the form of Hi-Loks

@CarloFelicione: That is absolutely right. With high-strength alloys and high hourly wages, hi-loks are the more economical alternative.

They are cheap and simple, especially in the volumes needed.
They are impossible to open, a useful safety consideration. You only have to check that they are properly inserted from the beginning. They won't shake loose.
Flush rivets are aerodynamically good since they can be made flush with the fuselage, which is difficult (or possible?) with bolts I believe.
(Blind) rivets are great for complex structures, since you can apply them entirely from the one side. You might be able to use screws if you pre-inserted them and held the thread and rotated the nut, but the nuts would still be outside the skin in the end.

Furthermore, I believe a protective coating is applied between many (commercial) aircraft panels for moisture and corrosion protection, so you wouldn't want to take them off to start with.
There's very seldom any reason to start taking apart aircraft. If it's bad, you can rivet over it or drill out the rivet.
Rivets can be made a bit smaller than bolts, saving weight. (Not entirely confident on this one)

Replacing airframe panels and inspecting hard-to-reach parts would be
  easier with screws, I presume.

Every time you make a hatch it introduces a weak spot into the loaded airframe, such as the skin covering the wings and the cabin, it has to be reinforced to bring it up to the required strength, adding weight. That's why these are kept to a bare minimum.

That's not to say aviation doesn't use screws or has its own versions. One screw derivative is called a hi-lok and is a permanent joining like a rivet. It works much like a classic bolt but has a calibrated diameter between the top hex nut and the bottom nut, so it snaps off when it reaches the correct torque. That way it's got a safety guarantee being attached well.

It's worth noting that rivets can "work loose" over time, though it's not as common as with screws in my experience. When a rivet works loose you frequently see a "smoking rivet" (with a trail of ground aluminum downwind of the loose rivet that looks like "smoke"). The fix is generally simple (re-buck the rivet and it tightens up).

"Cheap" for given values of cheap for aerospace certified components :D

@voretaq7: Its just somewhat scary if you find one of them inside a plane; though it is good for a priceless look of a stewardess if you give it to her after touchdown, asking to be delivered to the captain...

Saving weight sounds very right. Bolts are steel or titanium and have a bit sticking out to accommodate nut & washers.

WRT taking things apart, the places you might need to access generally do use screws, at least in my (limited to SEL) experience.

Vibrating loose is the major concern.
The panels are generally made from two sheets of aluminum skin (.030 - .060 thick)  with a paper honeycomb bonded between them and solid aluminum rails from 1" - 3" wide at the edges. This makes the panels incredibly resistant to flex or compression spread over the surface  and extremely lightweight. (You could easily, however shove a pencil through the middle of it (not recommended to try in-flight))
Using screws would  a) easily distort the surface of the panels (consider the way a screw is applied (torsion, which causes it to pull itself in) vs a rivet (which only applies force against itself until near-full compression is reached) or b) be highly susceptible to vibrating loose ( particularly since the skin is too thin to contain even a single thread )
It is possible to use screws into the solid edges, but you still have the risk of vibrating loose, plus the manufacturing costs of threading holes or using capture-nuts.

Could you elaborate on what kind of panels you are referring to? Those do not sound like standard paneling for airliners (no sandwich there).

think the BA BAC111 that had a window blow out because the wrong screws were used, causing the captain to be half sucked out of the aircraft, crew pulled at him to prevent him falling to his death while the copilot made an emergency landing...

Flush rivets are used on aircraft for aerodynamic reasons.
Rivets stand up to vibration better than normal screws.

What differentiates a riveted joint from a bolted joint is that the rivet expands when bucked to completely fill the hole into which it has been installed. Bolts/screws require that the pattern of holes be drilled very close to perfect locations, but still the required tolerance of the hole over the fastener size means that the joint must either be secured by sufficient clamping force to hold by friction, or one must accept that the joint must move to load the fasteners in shear. Of course, that means when the load reverses (as do many airframe loads) the joint can only load in shear after it has moved into the new load direction. Also to BE in shear loading, the bolt pattern has to be perfect, otherwise, loading of ALL of the applied shear joints can not be shared until the fastener or the structure has deformed sufficiently to allow the tolerance of hole location to equate and shared between all fasteners.
The next issue is weight, and it is another big one. Bolts are much heavier than rivets, and there can be thousands of them even on a small airplane.
Then there is load transfer:  To have an actual shear loaded joint with a threaded fastener, the threads should not be pressing on the side of the hole, so that means a washer under the nut so the shank can load against the hole - and usually a washer under the bolt head, as there is a radius under there that should not go into the hole unless there is enough thickness for a countersink for it to fit in.
Alternative: These problems can be (and are) dealt with in some situations by using countersunk screws, that take up the tolerance for shear loading by cocking slightly in their bore - but not really meant for serious shear loads but can give a good fit on removable panels.
Someone mentioned glued joints: common as dirt. The first Genav aircraft to do so was Jim Bede's BD1 - known to the rest of us a American Aviation Yankee (AA-1). Bonded joints are what made the skin of all AA1/A/B/C and AA5/A/B and AG5B as well as AG7 Cougars so clean that they fly a lot faster than their contemporary riveted competitors on the same power.

Nice answer - welcome to av.se!

Screws clamp parts together and shear stresses are transferred by friction between the parts. The shank of the screw will not participate in shear transfer.

When comparing rivets and bolts/nuts for fastening sheet metal, riveting comes out as the superior method.

Rivets are forged
From this site:

Riveting is a forging process that may be used to join parts together by way of a metal part called a rivet. The rivet acts to join the parts through adjacent surfaces.

Image source
The rivet is inserted in the pre-drilled hole, and then the ends are pressed or hammered together in order to expand the rivet into the hole. At the end of this process the rivet fills the hole snugly and completely, and the contact area helps in carrying stresses from one plate into the other. Clamping the plates together enables friction to take part of stress transfer as well. The forging process actually strengthens steel rivets.

Image source
Bolts don't fit
Bolting down two metal sheets works a bit differently: a hole is drilled in the sheets and a bolt inserted, the bolt usually having a smaller diameter than the hole. The bolt itself does not contribute to stress transfer, that is purely done by the friction from clamping. It is possible to drill a tightly tolerance hole and insert a bolt that fits exactly, but this is an expensive process that requires perfectly perpendicular drilling and care in handling. Expensive and slow, best done on straight surfaces, not the bent shape of an aircraft fuselage.

Steel and aluminium corrode
Bolts are loaded under tension and are best made from steel: aluminium bolts are stripped of their threads easily and stretch a lot. But if we use steel bolts on aluminium sheet, we get galvanic corrosion. We must either use steel on steel or aluminium on aluminium, and aluminium bolts are just not very good.

Nuts unfasten from fatigue
Nuts work themselves loose under alternating loads, and an aircraft has a lot of that. Every landing the wing slams down, bends upward again at take-off. The fuselage is pressurised at altitude, then de-pressurised at approach. The engines and hydraulic pumps introduce vibrations into the aircraft skin.
In order to stop the nut from unfastening, a locking washer needs to be installed or a compound such as loc-tite must be applied - at every one of the thousands of bolts that would fasten the aircraft skin plates.

Rivets are airtight
A point missed by the previous answerers: the fuselage is a pressure vessel, drill it full of holes for bolts, even tightly fitting ones, and it will leak like a sieve. Fill the hole up with forged aluminium and it will remain airtight.

Why is riveting not used in more applications then
Bolting metal sheets together has the advantage that the fastening can be undone without any specific tools. A rivet needs to be drilled out, a nut is just loosened. Great for performing maintenance, but in permanent constructions like bridges there is no need to unfasten bits of it. Long life aircraft do need their skin panels to be replaced every once in a while, but even that is a bit of a non-event with aluminium rivets, they drill out easily.

Why is riveting not used more then? It used to be, providing strong structures for bridges for instance. The method that largely replaced it is welding:

Only one welder is required, instead of at least two riveters for each side of the rivet.
No need to heat rivets to a red-hot state in an oven and haul them up to the point of application.
Welding is a line joint, riveting a point joint: stress tubes need to dodge the rivet holes and are concentrated around them, while they flow over the complete width of the weld into the next plate.

Bonding is even better
The best joining method for metal sheets is the one that joins surfaces: glueing, although for construction purposes the proper word is bonding. Fokker pioneered the use of bonded aluminium sheets in the F-27 and F-28: no holes, perfectly smooth skin, large stress conversion area with relatively low stresses. Not an easy method to implement: large autoclaves are required for the bonding process, and it is not possible to visually inspect the quality of the bond, like it is with a weld. For that purpose an ultrasonic bond tester was developed which would reveal bonding imperfections.

You know what is even better still? Bonding and riveting. The rivets take care of the points where peeling would start. Bonding alone is not prudent except in the few cases without peel stresses.

Rivet Joints have more shock absorbing capacity,
They absorb shear stress better,
They are more leak proof and
They are permanent joints.

Thanks for your answer! Can you provide citations or links backing up your statements?

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At what elevation is the airfield considered a high elevation field for takeoff and landing for 737-8?
At what elevation is the airfield considered a high elevation field for takeoff and landing for 737-8 ?

I don't believe there's a distinct threshold that anyone calls "high elevation."  There's a smooth continuum from at-or-below sea level, up to whatever altitude the manufacturer has determined is elevation where there's no longer a practical market demand.  There's not a magic weight, temperature, pressure, or field elevation that would officially start "high elevations."  I either have enough runway and climb gradient (including constraints for engine out situations) or I don't.

@MaxR great one, thank you.

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text:
Crop dusting a person's orchard and garden other than the field he is spraying
I'm a homeowner next to a farm field owned by a local farmer.  He has his crops sprayed several times a year.  This last time he sprayed the field I went and sat on my driveway, so he didn't get close to spraying my orchard and garden. So, he flew behind me came over my wood line and sprayed my garden, orchard, me and my truck. Then flew away.  I confronted his company in not a nice manner my fault. And got talked to from the local law enforcement saying they wouldn't press charges against me.  But the crop duster company denied they flew over my field and house but then called and talked to the local farmer, so their stories matched.  All pilots have to file a flight plan correct.  Is there any way to get a copy of that to file a complaint of them flying continuous over our property while dusting the neighbors field?  I have breathing issues so being sprayed by a chemical from a plane and having swollen eyes and trouble breathing is a big issue for me. We also have farm animals a Horse and goats.  But this doesn't seem to even bother this crop-dusting pilot.

Next time this happens, a video would be supremely useful.

Thanks I plan on doing that.

"All pilots have to file a flight plan correct?" False.  Most smaller plane, GA, VFR flights happen without a flight plan, and even if one is filed, there's no guarantee the pilot sticks to it.

Even if a flight plan existed, which it definitely won't in this case, and even if it had such precise detail as spraying patterns, which it definitely wouldn't, the pilot wouldn't put "overspraying neighboring plots" in it. Radar tracks, which wouldn't exist anyway due to the low altitude, aren't precise enough to prove overspraying. Photos or video are really your only hope.

It's not always clear who you mean by "he".  Sounds like you are saying the farmer is spraying his own crops?  Also you contradict yourself: in one sentence "he didn't get close to spraying my orchard and garden" and in the next sentence "he flew behind me came over my wood line and sprayed my garden, orchard, me and my truck.". --???

I will be better at how I word things the next time.  Since I talked to a person this morning about just that.

Yes, the person I talked to this morning told me the same thing most of you folks have also told me.  Video and pictures are my next best bet.  Getting the plane's Tail numbers also.  Since I'm retired now, I will be sitting on the porch with my video camera just waiting each time I hear those planes Fly.  and how arrogant that pilot was I have no doubt he will continue to do it.

If you preserve anything sprayed there should be residue which could be identified: clothes, the vehicle, plants from the garden/orchard. The pilot could lose his license for intentional behavior like that, and would likely at least get a warning if it were unintentional. Video of the property line would be good for future.

Assuming you're in the US, flights are only recorded by the pilot in his personal logbook (in Canada flights are recorded in an aircraft specific Journey Log as well). There won't be any records of flights that you would have access to unless you subpoena'd the pilot, and the pilot's own log book may be thin on the details anyway.
Your best bet is to make video of the action, making sure the airplane's registration can be made out, and take it to whatever agency in your state regulates pest control activity (it's a pest control application matter, not really a flying matter per se - you'd have the same problem if he was blasting you with chemical with a tractor).
They shouldn't be overspraying like that.  Operators mostly all use extremely precise GPS based pattern control systems for spraying, and aside from the potential health effects on the neighbours, they should be pretty careful about not throwing money way dumping chemical where it isn't needed anyway.  Sounds like a sloppy operator.

It's a guidance system that provides start/stop and track guidance.  Basically a stand alone micro-navigation system.  Pilot still controls the spray.  There's all kinds of vids on youtube that show these systems in action.


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Do private airports have to have the same runway and taxiway dimension and separation requirements as public airports?
I'm new to this site. I joined today after searching for hours to find answers to this question with no luck.
I am a part of a group of pilots and aviation enthusiasts working on a new aviation community development project. We have done extensive research and have collected a ton of data on various aspects of building an aviation community. However, we haven't had much luck finding definitive answers to the above question. I will elaborate more on the question...
Specifically, I would like to know if there are any requirements and/or regulations to follow with respect to the following for runways and taxiways in a private aviation community, or is it up to the discretion of the developers/owners to decide?

Runway width and length requirements
Runway Safety Area
Runway Blast Pad
Runway Object Free Area
Runway Shoulder
Runway Protection Zone
Runway Centerline to Taxiway Centerline separation
Taxiway width requirements
Taxiway Safety Area
Taxiway Shoulder

As I said, I tried for some time to research this, but all I can find is data for public airports. Nothing for private ones. If anyone can provide the answer or point me in the right direction, it would be greatly appreciated. Thank you.

Welcome to the site! It sounds like a great project, what regulatory jurisdiction is it in, i.e. what country?

Yes GdD, we are all very much excited about this project and can't wait to start the development and building process. Currently, we are about to make an offer on a parcel of land in the Caribbean. That is our first option as everyone in our group are retired or semi-retired and prefer the tropics. If that deal falls through, then our next option is in the US preferably Florida

757toga, thank you for the link. I will give them a call tomorrow.

I finally managed to speak to the right person at the Orlando ADO - a Mr. Scott Cararro. He was very helpful. In a nutshell, as far as the FAA goes, private airports are not obligated to follow any requirements as public airports do. He said the FAA is mostly concerned with the airspace above the airport, and what happens on the private property is up to the owners/operators and the FDOT. He did say however, it is highly recommended to follow the public airport requirements, especially if larger private aircraft like the G650 and G700 will be using the private airport. Thanks again 757toga.

In the United Kingdom, the document CAP 168 outlines the requirements you must meet to have your aerodrome licensed by the CAA. In other countries there will likely be similar documents.
The requirements for "public" and "private" (referred to as "ordinary" by the CAA) aerodromes do not differ.
If you don't want your aerodrome to be licensed by the CAA then you don't need to meet their criteria, but some insurers won't let you fly to/from unlicensed aerodromes.

Yes opr, I thought so too. But when I researched some of the existing aviation communities in the US, there was no consistency with respect to this topic. Some even had a parallel taxiway only 70 feet from the runway. It didn't seem like they were ahering to any regulations or guidelines.


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Why aren't airports built identically (like aircraft)?
Considering the delays and mishaps in super-projects (e.g. the new airport in Berlin), wouldn't it be rational to standardize airports in the same ways aircraft are: Building them all according to the same designs and not in a "one-off" fashion? Why is this not done?

airport layouts are influenced by the runway locations

Why aren't all hospitals built to the same design? Why not city hall? How about national parliament/capitol buildings? I don't think even the Soviet Union standardised public landmark building designs.

Many airports are built on old airfields, before people even had such ideas.  Wind, runway layout, local terrain, existing roads....

Local politics often trumps technical concerns.

Those who voted to close as too broad, how could this question be made more specific?

I'm not sure why this is considered too broad. There are already two excellent answers which suggests to me they did not think it was too hard to answer.

1. Terrain and land use
Several new airports are build on landfill island or coast next to mountains, their shape is influenced by the terrain and their size.
Hong Kong International Airport runways and shape are influenced by the terrain. Its curved east bank looks like the Chek Lap Kok island, which the airport is built on there. Both HKIA runways are parallel to the hill of Lantau island, south of the airport island.
HKIA is on the north of Lantau island (largest green shape), the grey colour stands for reclamation land and the green colour is the originally Chek Lap Kok island.

2. Prevailing wind
Prevailing winds influence the runway heading. Since takeoffs and landings are usually done as close to headwind as possible, wind direction generally determines the active runway. If the prevailing wind is uncertain, multiple runways with different directions will be built, Boston Logan airport is a extreme example.

3. Terminal building and bargaining power of airport management
This factor largely influences the terminal layout. In USA the airport management is largely influenced by the airlines, sometimes the terminal buildings are managed by airlines themselves, like New York JFK. The airlines prefer the terminal building which can minimize the taxiing time, passengers transfer time and maximize the perimeter (and hence the number of gates), Atlanta Hartsfield airport is one good example. Its linear shape can minimize the taxiing time and increase the number of gates available.

On the other hand, for main airports outside USA and Europe, the airport owner has strong marginal power of the airport management. They will choose the terminal building which can best fit for their strategy. Some airports owned by the government prefer a massive size of airport as an image of the country/cities; and some airports prefer large commons which can maximize the shopping experience (and hence rental income). Pictured below is Beijing Capital International Airport.

4. Historical factor
Several airports were built before World War II and some even existed before World War I. At that time the airport was just a new concept. A full development and operation methodology for airport had not been developed. Many full airport layouts, which nowadays seem to be "inefficient", were developed. And since the layout with the buildings were built, it is costly and time-consuming for reconstruction, especially at the busiest airports. Airport managers tends to expand the existing building with new gates and other facilities, rather than total reconstruction. Pictured below is a master plan for O'Hare International Airport from 1948.

Heathrow airport is reconstructing Terminals 1-3, replaced by linear buildings like Terminal 5. It will take 5 years to destroy the old Terminal 2 and phase one was just completed last year. Terminal 3 will be rebuilt after 2019 and the whole project will take more than a decade.

5. Airport operation
Operations of the airport are influencing the airport layout. For airports have more flights for both general aviation and airlines, they tend to have more runways, like Dallas DFW and Amsterdam Schiphol. On the other hand, in most parts of the world there are only 1 or 2 runways even for the most busiest airports where most of the flights are from airliners, like Dubai and Hong Kong, ranking for 6th and 10th busiest airport in terms of total passengers.

6. Reversal: Experience
On the other hand, huge number of airports has been built in the half century, several airport design has been well developed. We can see the latest airport may share similar terminal shape, runway plan and airport layout.
Hong Kong international airport and Hamad International airport

Chongqing airport (Far future) and New Lisbon airport proposal

Conclusion
To sum up, there is not one optimal conclusion of airport layout. The airports are built related to physical limitation, historical constraints, stakeholders opinions, and, of course, the architects' designs. No airport will be identical, regardless the layout, terminal shape and style. On the other hand, the experience from airport planner will create a similar airport.

This is a good answer! I would note, though, on the last point that very busy airports with only a few runways are usually due more to space constraints than to the presence of GA traffic. For example, DFW (that you mentioned) actually has only 1% GA traffic. It's almost exclusively airliners (99%, according to the FAA A/FD.) Dubai is building a new airport that is planned to have 5 parallel runways. Their current location just doesn't have room to build more.

Another reasons for airports to need more runways, of course, is number of movements (i.e. takeoffs and landings.) Going back to the DFW vs. DXB example, while DXB had 10% more passengers than DFW in 2014, DFW had 90% more movements than DXB in that same time period (due to most of the movements being domestic operations on regional jets, 737s, MD-90s, A320s, etc. rather than long-haul operations on 777s and A380s like at DXB.)

@ reirab but even 3-4 runways are sufficient for the most busiest airport

It's sufficient for most, but not really for O'Hare or Atlanta. Both of those airports had about 2.5x as many movements as DXB last year. Even with 5 runways, Atlanta can get backed up pretty quickly.

Please provide attribution for all of your images.

At Logan, the prevailing winds can be from all directions, often on the same day!

You can standardize a procedure to determine the optimal airport, but this wont produce a constant shaped airport.  The biggest factors in airport construction are the runways.  Runways are influenced by

prevailing winds
anticipated operations (volume and type)
surrounding terrain

If there is only one dominant prevailing wind direction you can get away with runways in one direction.  If you have a significant secondary prevailing direction, you'll need a crosswind runway.  If you want to handle lots of operations, you need multiple runways in the same direction.  To maximize operations you want them to be widely spaced apart.  If you want to handle large airplanes you need longer runways.
All of those factors (and more) will determine an optimal runway layout, but practical considerations (mostly finite available space) will turn the optimal layout into something else.
This procedure means a one-airport-fits-all solution isn't very realistic.  This also only applies to new construction (see: KDEN).  Many airport construction projects are also limited by the existing airport they are expanding and the development around them.

It's not that airports are not standardized -- in the US, FAR part 139 describes the standards that your typical significant airport must meet.  It's that they must be tailored to fit the local conditions:

usage mix (business GA, flight training, cropdusters, heavy jets, military fighters, aircraft factories, and more)
topography (runways have the most stringent standards on topography of any transportation facility found on land, even more stringent than rail lines, and also can't bend around corners)
wind direction (especially important if you are in an area with variable weather and/or handling small planes with poor crosswind capability)
weather of other sorts (do you need extra long runways due to "hot and high" problems such as Mexico City? Do you need to be able to keep runways clear of extreme snow and ice fall?)
operational capacity (multiple primary and even multiple parallel crosswind runways, accompanied by complex taxiway networks, are needed at large airports in order to keep them from becoming total bottlenecks, see SAN or LHR for what happens when you can't put more runways in)
obstacle clearance (instrument approaches need significant swathes of clear airspace in straight lines unless you are willing to limit your customers to Part 121, 135, 91K, and business 91 operators with very recent avionics capable of flying Radius-to-Fix (RF) legs, and these requirements become more and more stringent as the weather gets worse.  Phoenix can get away with having obstacle issues because the weather doesn't exactly turn rotten there very often, but say Chicago O'Hare, Heathrow, or St. John's needs to be able to get planes in even in truly terrible conditions of thick fog, swirling snow, and driving rain)
and landside connectivity (major passenger facilities will need gobs of parking and major highway connections, as well as hooks into public transit, while a cargo hub or aircraft factory will need the ability to handle heavy trucks or even shipments by freight rail, and there's also the need for fuel...)

As a result, the standards in part 139 and the various documents that implement it must be quite flexible to adapt to the varying needs of the NAS' airports.


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Why are the top speeds for jet engines higher than for propellers?
I realize turbines are more efficient than piston engines, but if that were true, then why don't turbo-props reach jet speeds?

Related: Can propeller driven aircraft sustain speeds at or above Mach 1?

Related: Why is the Tu-95 so efficient despite having propellers that spin faster than the speed of sound?

The thrust of a propeller is proportional to the inverse of airspeed, while the thrust of a pure turbojet is roughly constant over airspeed in the subsonic region. This means that two airplanes with the same static thrust, one propeller-powered and the other jet-powered, will reach very different top speeds.
And, no, piston engines are more efficient.
A piston engine produces a constant torque $\tau$, independent of speed. This torque drives the propeller which produces work $W$ per unit of time on the air passing through it. This work per unit of time is power $P$ and proportional to the product of torque and propeller speed $\omega$, which is again constant over airspeed $v$. The power to propel the aircraft is the product of thrust and airspeed and equals engine power times propeller efficiency $\eta_{Prop}$. When airspeed goes up, thrust must go down proportionally for power to stay constant.
$$P = \tau\cdot\omega = T\cdot v\cdot\eta_{Prop}$$
$$\Rightarrow T \varpropto \frac{1}{v}$$
Turbojets, on the other hand, profit from flight speed because the intake pre-compresses the air when it slows down ahead of and in the intake. This pre-compression lifts the pressure level of the whole engine, so it sees an increased mass flow with increasing speed, producing higher thrust. This effect by itself would increase thrust in proportion with the square of flight speed, but the same effect which reduces the thrust of a propeller acts on a turbojet as well. This effect, however, is less pronounced because the jet accelerates less air to a higher speed, and both cancel each other, roughly.
Turbofans are more similar to propeller engines, so here the thrust goes down over airspeed, and more so for higher bypass ratios. Turboprops are even closer to piston-powered propellers, so their thrust drops even more with increasing speed. This means that the speed at which drag equals thrust drops when you move from turbojets to turbofans, and further to turboprops and is lowest for piston-powered propeller aircraft.

"The thrust of a propeller decreases with the inverse of airspeed", is this 'double negation' on purpose?

@ROIMaison: Thanks for pointing this out. I was thinking of two ways to express it and mixed them up. I guess it is now clearer.

it is clearer now :)

For a turbo prop the limiting factor to some extent is the efficiency of the propeller, not the engine. There have been a bunch of attempts at very fast turbo props over the years with the fastest most likely going to the XF-84H Thunderscreach but in all of the cases the propeller becomes the limiting factor.
First off, to get going in the speed range you mention with a usable size propeller your tips will go supersonic which presents problems into its self. The XF-84H suffered from this and you can read up on why that was a noise issue.
On top of the noise issue it simply becomes more efficient to use a jet at those speeds so in practice its the power plant of choice. Many of today's top preforming turboprops easily encroach on the speed of light jets.

Haha, I just answered a question about dual turbines powering a single rotor, citing the XF-84H yesterday.   Constant visible shock waves emanating from the prop area outward, and the ability to give people seizures.  The fact that it also got thrust via the Allison XT40's exhaust, and had a (never used) afterburner, is just crazy


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Is landing without engine power routinely feasible?
Considering that airplanes are very noisy, could one routinely land them with the engines turned off, gliding onto the runway? Could the engines be started fast enough for an emergency situation?

They could but the engine at idle you can't even hear it probably. Even an airliner isn't all that loud on approach. Anyway it's not really all that safe since most approaches start from further out than a power-off glide so the whole way pilots approach the airport would have to be changed to a "Baghdad" type approach like a glider pilot would use circling in from above.

Much of the noise is from the flaps rather than the engine.  Turning off the engines lowers safety a great deal but only lowers noise some.

Brin, could you provide a source for flaps causing noise? I’m having a hard time imagining how that would work. I know that on a small plane the position of the prop can affect the amount of noise, but I have never noticed any change in noise level when extending the flaps. There is some cabin noise in jets from the motors used to extend the flaps but I doubt if you can hear it on the ground.

Not feasible. A start of each engine can take 30-60 seconds or longer and would require the APU (noisy). In some airplanes you lose the packs during start so you would impact pressurization. Additionally many engines have a required time at idle after start before significant thrust is applied.  Lastly, with all engines offline, the airplane will be in some sort of essential power mode and many systems will be offline or unavailable.
You might then ask that we land at idle but that can present issues with spool up time before thrust is available.
One of the primary reason we land with a high flap setting is so that we have the drag to maintain approach speeds with the engines spooled up. This gives us near immediate power if we need it to go around.
I'm sure someone else can provide historical data from when idle approaches in jets were normal and specifics into why we don't do this today.

Also many aircraft use thrust reversers to reduce the ground roll, no power means no thrust reverse and longer ground roll on landing. Next time you're out driving and approaching a set of traffic lights turn your engine off before you're stationary. How much harder is it to stop? Obviously don't try it for real, it's not safe, but hopefully you get the idea.

@Adrian I didnt mention that since landing performance data assumes no reverser use. More problematic with all engines out in the EMB-145 is that you lose flaps and pressurization, and lots of instrumentation. We needed 3 generators online for full electronics and the APU only had one.

Pressurization on final?  And the downside of the flaps dragging / engine at power approach is if you lose engines, you now are unable to finish the approach in glide.  The first 777 loss had that problem IIRC.

Is it possible to land them with the engines off routinely?
Yes. Depending on the glide ratio of the aircraft, you most certainly can land them. And in flight training, we do practice this routinely, although we don't actually turn the engine off for safety reasons, merely reducing the power to idle. Technically, this is not the same thing, as the propeller is still turning and producing thrust, and of course the engine is on, producing noise, but it is certainly good to know how to land without an engine.
But I suspect you really were asking about the more important part of the question - should you, and is it safe?
Well, no, not really. You can check out this list of flights that required gliding. You'll notice that several incidents that required gliding were very succesful, and they glided for quite a long way. However, most notably for your question is the first flight on the list - a DC-3 on approach lost its engines and attempted to glide in. They failed to clear a mountain, and crashed, killing everyone aboard. If you were gliding into an airport, and suddenly needed power, it would take too long to restart.
Additionally, if weather conditions caused your first approach to be off, it would be definitely better to 'go around' than attempt to save a bad landing. Just watch the first landing of this video. It would not be possible to restart and engine before touchdown in that situation. Maybe it could be done earlier in the approach, but as the landing is a task-saturated phase of flight, you do not want to attempt it in those situations (in my training, we do not attempt to restart an engine under 2,000' AGL).
So in summary, yes it could be done, but no, you absolutely wouldn't want to do it because of the potential for needing instant power.

Could you possibly clarify what "800' AGL" means?

Sure. 800 feet above ground layer. So 800 feet above the ground, as opposed to 800 feet above sea level

@SSumner - What's a ground layer?  I thought it was ground level.

@SteveV. - oops. I had a brain fart there. You are correct

"However, most notably [sic] for your question is the first flight on the list - a DC-3 on approach lost its engines and attempted to glide in. They failed to clear a mountain, and crashed, killing everyone aboard."  That list gives a total of 7 fatalities out of a total of 25 people onboard, and, last I checked, 7 < 25.

The other answers describe well why engine power is necessary during approach.
I would just like to add that powering off the engines would not decrease sound levels as much as you would expect. Indeed, other devices such as landing gears and high lift devices produce noise (especially slats) on the same order of magnitude as the engines and those are obviously necessary during landing.
Edit:
It has been pointed out that I should back my claims with references and I agree that it is a good habit to take. Unforntunately, I have to admit that I only had some faint memories of a poster I had seen somewhere. Some googling led to this study [1] which provides some hard data on the A340. I am not familiar with the study but seems that they have managed to identify different noise sources and their related noise levels as follows :

Engines ~ 130 dB
Landing gear ~ 127 dB
Slats, flaps ~ 124 dB

So the engines are only 3 dB louder than the landing gear themselves just 3 dB louder than the slats and flaps. (It would seem that, from the ground at least, slats are only marginally louder. So I was wrong about that, huh!) In another study [2] (that I am also unfamiliar with), it appears in their recommendations that the removal of a noise source only 3 dB louder than others will only result in a 3 dB reduction of the overall noise.
[1] Sijtsma, Pieter, and Robert Stoker. "Determination of absolute contributions of aircraft noise components using fly-over array measurements." AIAA paper 2958.2004 (2004): 10.
[2] Lockard, David P., and Geoffrey M. Lilley. "The airframe noise reduction challenge." NASA/TM 213013 (2004): 2004.

May I point out that 3 dB of difference means double the noise and 6 dBs are 4 times it? The scale is logatithmic!

@rookiecoder in acoustics the decibel level is defined L = 20*log(I/I_ref) therefore a 3 dB change results in a 10**(3/20) factor in intensity, a 41 % increase. That's not double. However, that is also not how we perceive it... For example, the pain threshold level is one trillion times that of the sensitivity threshold but it doesnt seem like it. That said, I'm no specialist that's why I did not make any conclusions.

I think that the nasa paper is public, you can directly link it in your answer: http://www.cs.odu.edu/~mln/ltrs-pdfs/NASA-2004-tm213013.pdf

There is one thing that previous answers completely missed. It is the simultaneous controlability of descent slope and descent speed. Any glider pilot (I am one of them) knows that there is a strong relationship betweeen slope and speed. This is why almost every glider have airbrakes, they allow some independant control of both parameters. For this the approach in a glider is made at a glide slope near 10:1 although except some very old gliders most of them have a best glide ratio higher than 30:1, which would be completely unpratical for landing. The combined action on airbrakes and pitch attitude with the elevator can be used to change airspeed on a fixed slope or to change slope while keeping the same airspeed, as long as the slope is high compared to best glide.
On a powered aircraft this control is obtained by adjusting power and pitch attitude. This requires the opposite to the glider case : the approach should be on a slope less than the best glide slope, the standard is 5%. This corresponds to nearly the best glide of a liner, so no correction would be possible. Of course an unpowered approach could and should be on a higher slope, but the lack of independance between slope and speed remains.
I know of 2 cases where the problem occured.
The fist one is known as the Gimli glider, a Canadian 767 that went out of fuel due to some miscalculation on the fuel at takeoff and landed on an unused airfield using slip instead of airbrakes to adjust the glide slope, as was done in older gliders with unefficient or no airbrakes, one of the pilots was an experienced glider pilot knowing this technique.
The second one is probably the famous case of Sullenberger landing an Airbus on the Hudson river. Some people argued that he was in gliding range of two airfields when he took this decision. But the lack of possibility of precise control of the approach slope toward a runway with high buildings in the vicinity made probably the Hudson a better choice.

The start-up sequence of some engines includes a period of increased drag.  This drag can ultimately crash an aircraft that is already short of power and trying to start an engine at a low altitude to get extra power.  This example was a turboprop.  The effect is caused by propeller positioning optimized for minimizing drag while in the off mode.  Turbojet engines might not have this problem but they still take very long to start, and in fact, even to boost power in an emergency.


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Why is the seat belt mechanism on airplanes so different from the the one on cars?
You pull a flap on airplane seatbelts, you push a button on car seatbelts. Why?

There are actually airplanes with the same type of seat belts you find in cars. Not the other way around that I know of though.

They're easy to unlock, and it's easy to inspect visually by flight attendants if they are closed. Not sure whether that is the reason.

Before cars had push-button seat belts, they had the kind with flaps you pull.

@DeltaLima; I have absolutely no idea, but in my personal opinion, I feel lifting the buckle-cover is a lot easier to open. You'd be amazed how many people have trouble opening seat belts in a panic, I'd imagine the push-button style would be even worse.

Are you talking about a commercial airliner or a GA plane? On a Cessna 172 (made in 1998), the seat belt is very similar to the ones in cars.

Related: Some elements in this answer.

As Roe pointed out you don't necessarily pull a flap on airplane seatbelts -- Sometimes (particularly in newer light GA planes from the 1990s and on) you push a button just like you do in a car, other times it's the "utility buckle" (flap buckle) - sometimes with a shoulder harness attach peg or other attach mechanism, and sometimes you'll find rotary buckles, where you turn a knob to release the belt (particularly common in aerobatic aircraft, with 4-point or 5-point restraints).  I've attached a little gallery of belt types below (and this isn't even complete - military aircraft hafe some downright funky buckles!).

This variety is why the FAA requires flight crew to instruct passengers in the operation of the seat belts.

So, why are the belts in airplanes different?  And why are there so many types?
Each belt is good at something different, and each belt has its own set of drawbacks.

The automotive style push-button belt is very familiar to infrequent flyers.
Most people who are going to get on a plane have been in a car (they probably rode in one to the airport), so familiarity with automotive-style seatbelts can generally be assumed. This is great when you take someone for a flight who isn't a frequent airline passenger.
The drawback to the automotive belt is it's not as easy for rescuers to operate, and the push-button mechanism can jam - particularly in fires where the plastic shell can melt/fuse (in cars the belts are often cut by rescuers because it's easier/faster than dealing with the button). If your passengers are injured in an emergency and can't operate the belt it may be difficult for you to locate and press the button to assist in evacuation.
The "utility buckle" flap belt is easy for rescuers & cabin crew to operate.
The exact opposite of the push-button belt's chief drawback: It's really easy for cabin crew or rescurers to find a utility buckle on a belt by feel and yank the flap to release it (try it some time: Buckle someone into an airplane seat, close your eyes, and find/release the buckle. The hardest part is finding the belt if they're wearing bulky clothes). It's also relatively hard to jam a utility buckle, and relatively easy to inspect them to determine if they're damaged.
The drawback to the utility buckle is that not everyone has been on an airplane (or in a really old car that uses them), so they might not be familiar with "Lift the flap to release the belt", and in a panic situation it may take them a few seconds to remember what to do.
The rotary buckle is unique and kind of awesome for what it's built for.
Rotary buckles make it easy to attach 4 or 5 point restraints, and to release all or part of them quickly.  (The buckle shown in the photos is single-function - turn to release - but dual-function buckles exist where you can press the button in the center of the buckle to release the shoulder straps, or turn the knob to release everything.)
The big drawbacks to rotary buckles are their novelty: On the ground you usually only find them on 5-point racing harnesses, and in the air they're pretty uncommon too (mainly aerobatic aircraft as I mentioned earlier). They're probably the last type of buckle a passenger or rescuer would expect to have to deal with, and particularly with the dual-action buttons they can present a challenge in a panic egress (passengers will hit the button and release the shoulder harnesses, but they're still restrained by the lower portion of the belt - this can cost precious time for them to realize they have to turn the knob).

What's the fifth point of attachment in a 5-point harness.

@curious_cat A standard 5-point harness has two shoulder straps, two wast straps (the standard "lap belt", and a "crotch strap" to prevent submarining

I have even seen planes with double seatbelts. On one aerobatic glider there was the regular 5-point harness, and a separate lap-belt that went over it. The owner told me there had been situations where someone had accidentally opened the seatbelt during an aerobatic maneuver, causing the pilot to fly out of the glider.

Excellent analysis. A slightly OT note: The "utility buckle" flap is no longer acceptable in cars because in a high-impact stop, the G-force of stopping can easily over come the strength of the spring holding the flap closed, thus the buckle can open exactly when you least want it to. This isn't an issue with the side mount push button type since you push down on it. Some older side mount one had a button that you pushed in (toward yourself) that could be released in a side-impact crash.

Also, if you're going to test out how easy it is to find and release the "utility buckle" with your eyes closed, be sure you're testing on someone who doesn't mind you groping around in their lap with your eyes closed. :)

Actually, the 5-point harness with rotary buckle tends to be present in most aircraft —the pilot seats in transport aircraft usually do have 5-point harnesses.

The "utility buckle" is both easy to inspect and easy to repair; it's harder to know when a push button one goes bad since the mechanism is sealed, and when (not if) it does, you pretty much have to buy a new one. For an airline with thousands (millions?) of seat buckles to maintain, the choice is obvious. For individual owners, whether cars or GA, the choice is different

@Masse: Hope that pilot was wearing his parachute...

I think the classic economy class buckle is just a compromise: On cars, you can easily reach the side of the seat, and it's more comfortable to have it on the side than a block of metal in the middle. In aircraft seats however, it's difficult to reach down in between, and if something happens quickly, you want to easily release it. The design itself might have a historical element, since a seatbelt which is very easy to open might make passengers feel safer, especially when many were new to flying.
That being said, there are several business-class seats today which utilize conventional seatbelt styles.

When you're sitting in a row of 5 seats, it could be confusing to figure out who you're really unbuckling if the push-button is on the side of the seat. It could be you, but it could also be your neighbour :) Less confusing if the mechanism is on your own belly.


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At what point during a long flight do commercial airliners have the best gas mileage?
This question has several parts that are interrelated, and I didn't want to split it up:
Considering that several tons of fuel are consumed during a long haul flight which requires lift to be reduced as the flight progresses, do pilots do this by changing wing geometry, altitude, speed or all of the above?
Is this done automatically or manually?
What is the optimal strategy if you want to minimize fuel costs?
Having chosen the optimal strategy, is the latest phase of the flight where the airplane weighs least necessarily the one that gets the best gas mileage, or is there a minimum at a certain intermediate airplane mass during the flight?

I'm pretty sure you get the best mileage during the descent at the end of your trip, but that's not really what you're asking, is it? :)

I also believe this is probably the case, but am not completely sure: Maybe the reduced lift demands an angle of attack that has a poor drag, or the engine has to run at a power level that is suboptimal. However, I presume the relationship between wing configuration, altitude and engine power would be quite difficult to optimize during every point in the flight.

Descent is most efficient.  Lift and angle of attack are the same in the descent as in cruise (provided speed and weight are the same).  The only change is less power from the engines.

@Ralgha Lift is less during descent than cruise, pretty much by definition. If lift were the same, you wouldn't be descending!

No, lift is the same.  If lift were less you'd feel light in your seat, and you don't, unless it's an unusually sudden pitch down.  Stable climbs and descents are caused by excess or insufficient power.  Thinking that lift is changing is a common misconception, similar to the "downwind turn" misconception.

There are a lot of things that go into determining the most efficient way to operate a jet, and minimizing fuel does not always make it cheaper to operate.  Some general "rules" that are used are:

There is an optimum speed to fly the airplane, which depends on several factors:

Altitude
Fuel savings by flying slower -vs- increased maintenance costs because you are taking longer to get there (and putting more time on the airframe/engines).
Temperature at cruising altitude
Winds at cruising altitude

There is also an optimum altitude to fly the airplane:

Aircraft weight
The higher that you can fly, the better - until you don't have enough power to maintain your optimum speed at the optimum power setting for the engines.
Temperature at cruising altitude
Winds at cruising altitude
Altitudes that are available from ATC
The amount of fuel and time that it takes to climb -vs- the amount of time that you will have at the high speed cruise

Anything that makes the airplane heavier makes it unable to climb as high (and lowers the optimum altitude for the current conditions), and makes it burn more fuel.  This includes the fuel itself.  Assuming a constant speed and constant weather conditions, you will always be more fuel efficient later in the flight.
As fuel gets burned off, the amount of thrust required to maintain the optimum speed goes down and the optimum altitude goes up.  Basically, we keep reducing the power to maintain speed until the optimum altitude increases to an altitude that ATC can let us climb to.  This is known as a step climb.  The speed will be controlled manually by the pilot, or can be controlled automatically by auto throttles.
Today's sophisticated flight planning software takes all of this into account and gives the pilot a very good plan on how to operate the airplane to be the most efficient!

Actually, best cruise is flown at a specific lift coefficient, which in turn depends on the engine type, aspect ratio, zero-lift drag and wing efficiency.

@PeterKämpf You left this comment a long time ago, but I just saw it. "Best cruise" doesn't always result in the most economical flight, and for a given design (where all of the things that you mention in your comment are constant), and operating rules, these are the operational considerations which come in to play when trying to minimize total overall cost.

To give an example of the weight vs altitude capability for an old airplane, a 747-200 taking off at max weight isn't going to be able to climb higher than FL290 or FL310, and they may stagger to get to the latter. If the flight is in the neighborhood of 12 hours (which generally means a passenger aircraft as opposed to a freighter) FL390 or FL410 become usable. The aircraft is capable of FL450 if unusually light.
Though our old flight plans gave us the points that we could expect to be able to climb, there was a rule of thumb (which I unfortunately have forgotten) using the EPR that we used that quickly told us how many thousand feet above our current altitude we could get to.


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Does baro altitude from ADSB represent height above ground level or height above mean sea level?
I am retrieving ADSB data from Opensky-network.org. I would like to know what the baro altitude represents, is it the height above ground level or above mean sea level ?

I don't know what post-processing is done by the opensky-network, but the altimetry transmitted by ADS-B from the aircraft is the pressure altitude. This is the pressure measured by a static port on the aircraft, converted to altitude according to the formulas or tables of the ICAO standard atmosphere.
The ICAO Standard Atmosphere is hydrostatic atmosphere model which assumes a sea level pressure of 1013.25 hPa, a sea level temperature of 15 degrees Celsius and a lapse rate of -6.5 degrees / 1000 meters (up to 11 km, then the temperature is constant up to 20 km) .
The pressure altitude is equivalent to the Flight Levels used in air traffic control. A flight level is equal to 100 ft of pressure altitude.
The pressure altitude is not equivalent to the height above ground. The aircraft's altimetry system has generaly no way of knowing where the ground is (radar altimetry is not standard on every aircraft and is not connected to ADS-B).
The pressure altitude is also not equivalent to the height above MSL, because the assumptions of the ICAO standard atmosphere are typically different from the actual atmospheric pressure distribution.
ADS-B does also transmit geometric height, which is GPS derived and is expressed with respect to the WGS84 ellipsoid.


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RPM of wheels on touchdown
What is the average RPM of the wheels of commercial airplanes upon touchdown?
I understand there is variability that comes with different wheel sizes, approach speed, etc. Just thinking of the average RPM that the wheels almost instantly hit on contact.

At the instant of touchdown the wheel rotational speed is zero.

For a 737 the tyres are 27 inches in diameter and landing speed is about 145 knots.  Converting to SI we have a wheel circumference of 2.15m and a speed of 75m/s so that equates to about 35 revolutions per second or just over 2000rpm.

+1 for SI Units

Is 27 inches the tire, or the rim?  Most American cars have >27" diameter tires.

@Harper that’s 27” tyre.  Back in the old days wheel diameters for cars were around 14 inches.  737 tyres are 27x7. 75 R15 but I suppose an operator could fit 205/55 R18 with some fancy mags and an appropriate STC.

It’s a thing, apparently.  https://www.dunlopaircrafttyres.co.uk/aircraft/boeing-737-next-generation-tires/

The nosewheel is 27" diameter.

A typical heavy airliner has main tire diameters on the order of 50 inches, which works out to be a circumference of 157 inches, or about 13 feet.
A typical airliner might be moving at 125 Kts, or 12,500 FPM early in its ground roll.
The tire spins 1 rotation every 13 feet, so to cover 12,500 feet in one minute:
12,500 / 13 = 955 RPM.

Fair to say 737 tyres are relatively small so our estimates are probably towards the ends of the spectrum.


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What happens when you put significant weight on the tail of a commercial airplane?
I'm curious what happens if you put some extra weight in the back of the airplane (let's say a small GE turbine or something). I'm new to Stability and Control side of things and curious how the new moments can be counteracted and if we need to add extra weight to the airplane in order to counter-balance it. So far I'm of the understanding that if you add, let's say 1000lbs to the tail of the airplane, you'll probably need to add thrice as much weight close to the center of the airplane to balance it. I'd appreciate some info and help! Illustrations might help too :)

1000lbs is five 200-pound guys.

The answers to this question should help your curiosity. If not, "longitudinal stability" is the keyword to be searched: you'll get a lot of useful and interesting answers.

Basically you seem to be asking "what is the effect of a major aft shift in the CG".  That's a pretty broad question, but the answer would usually be "not good".

If it's significant weight, you have problems on the ground: https://thepointsguy.com/news/airplanes-center-of-gravity/

If you put extra weight in the tail of the airplane, the pilot then has to add extra downforce on the controls to pull the tail up and push the nose down so the plane can fly level. In doing so, there is now less control authority remaining to make the plane descend on purpose and beyond a certain point, the plane becomes impossible to properly control and hence unsafe to fly.
There are several spectacular youtube videos of cargo planes in which the cargo comes loose during takeoff and all slides into the tail of the plane, tipping the nose of the plane violently up and causing the wing to stall. The plane then basically falls out of the air with its nose pointing up and crashes into the ground tail-first.
Mayhem ensues.

It's fairly simple. The airplane is certified to fly with a center of mass, or Center of Gravity, that is between point X and Y, or a Forward and Aft limit.  You have to load it so the C of G falls in that range.
If you add weight at the back, it shifts the C of G back toward the weight you added some amount, depending on the weight relative to the total mass, and the distance away the weight was added. If the weight you added didn't move the C of G out of the allowable range, and you don't exceed the maximum total weight of the plane, you are good to go.
If the weight you added did move the C of G out of the allowable range, you have to add compensating weight on the opposite side of the C of G until it's moved back into range.  How much and where you add that weight doesn't matter as long as you move the C of G forward into the allowable range, and you don't exceed the maximum all up weight.  1000 lbs added to the tail 50 ft from the C of G would require 1000 lbs added forward 50 ft from the C of G to keep the C of G where it was originally. Or some smaller amount if it can be added more forward than that.  You're just talking weights, levers and moments in the end, why it's called Weight and Balance.
As far as what the limits of the C of G are, the aft limit is a function of static stability margin. The center of mass, the rotation center of the body in a fluid, has to be forward of the aerodynamic center of the entire body as a whole, the Neutral Point, the distance between the two points being the static margin.  Think of a weathervane held sideways, with the pivot of the weathervane equivalent to the C of G. The weathervane "trails" into the wind with the aerodynamic center (the Neutral Point in the airplane's case) behind the pivot axis (the C of G in the plane's case). The airplane weathervanes about its C of G, as the Neutral Point wants to trail behind the C of G in the airstream. It's statically stable.
Moving the C of G too far aft moves it closer to, at, or beyond the Neutral Point. If you move the C of G to the Neutral Point, it simply loses its ability to point into wind. Move it aft of the Neutral Point, and it wants to flip around and point the other way, because its static margin, the C of G to Neutral Point relationship, is now in that direction.
Trimming forces generated by the elevator surfaces provide the ability to make the airplane more than a statically stable lawn dart when it moves through the air. Trimming forces allow the "weathervane" to align itself at angles to the flow off of zero, so that lift can be generated by the lifting surfaces (held at a positive Angle of Attack) to hole the whole thing up instead of just proceeding in a ballistic arc.  It's the same as bending the trailing edge of a weathervane to force the weathervane to point itself into wind with its axis offset from the airstream, that offset being its Angle of Attack.
The forward limit is a function of the ability of the tail to generate trimming downforce in the most extreme-demand case, which is usually raising the nose for takeoff and landing around the fulcrum of the landing gear's contact patch with most tricycle gear airplanes, or a just desirable minimum flying speed (maximum AOA) in the case of a taildragger.
So, too far aft and the thing becomes a handful to fly as it no longer wants to "point" into the airstream, or in the extreme case wants to point in the opposite direction if you let it. Too far forward and your minimum flying speed, or the takoff and landing speed, will be higher than the airplane's specification.
Because of this, exceeding forward limit (within reason) is less dangerous than exceeding the aft limit.


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How does in‑air refuelling work from a strategic point of view?
There's plenty of articles explaining how in‑air refuelling works from a technical point of view, and this is not what I'm after.
What I'm struggling to understand is what's the point to begin with.
"To extend the range"
Yes but they extend the range how exactly?
If you go from A to B through R, you either have the range to go straight from A to B (which is not the case apparently), or you need to refuel when you approach R, which is halfway.
But this implies that you already have a base in R, with a tanker available.
So, if you already need to have a base in R, and it needs to be protected… what's the point then? Can't the operation just start from R instead of starting from A?
"Yeah but I don't have fighters ready in R"
Ok cool, so why doesn't your enemy attacks R, which is unprotected, preventing you from refuelling in the first place?
There has to be something I'm missing, what is it?

Related - https://aviation.stackexchange.com/questions/12671/why-is-mid-air-refueling-worth-it-or-is-it-at-all

The British RAF developed the first dedicated military aerial refueling system, for the top secret Lancaster task force that was created as a backstop to drop the atomic bombs in case the B-29 modification program was a failure.  Absent the modified B-29s, the Lanc was the only aircraft in the Allied air forces that could carry Fat Man and Little Boy.  But of course it didn't have the range, so a tanker mother ship was developed and tested and would have allowed them to make the trip from China.

But what if there is no "R"?

Take a look at the Black Buck missions that the RAF undertook as part of the 1982 Falklands War - two bombers, 11 tankers, 6,600 nautical miles, one base.  Without refuelling, those missions could not happen.   The tankers refuelled each other and the bombers to get them on target. https://en.m.wikipedia.org/wiki/Operation_Black_Buck

https://www.youtube.com/watch?v=jcEhZ2p4s9o

I recommend this puzzle: https://www.youtube.com/watch?v=dzrwnwOx0fw Your maximum fuel capacity only allows you to reach halfway to your destination. Yet with 3 aircraft able to refuel each other, you can do the full journey with one aircraft, using only one starting location for all three, with the end result being one plane doing the full journey, and the other two returning to the base. In real life it's even better, because tankers have a much longer range and carry more fuel. Fighters don't have all that much fuel capacity because they need the agility to fight, and to carry more ammo.

Consider when “R” is in the middle of the ocean. Or multiple points in a chain across an ocean. Carriers are only so big (no bombers), they take a long time to get into position, and they’re rather attractive targets if within non-refueled range of the enemy.

Wouldn't it also be true that a lighter aircraft uses less fuel? In which case you keep your fast fuel inefficient aircraft (jets / bombers) flying on minimal fuel, whilst your workhorse slow efficient powerful aircraft carrying all the weight of the fuel. Then they just top up the inefficient aircraft as and when required with the absolute minimum required for it to safely get out of any situation?
The fuel carrying aircraft in this case are akin to a mule/packhorse carrying all the bags slowly up Everest, so the weak humans can concentrate on getting up the hill.

If you check certain flight tracker websites, you will quite often see fighter jets and tankers flying in formation across the Atlantic. The refuelling aircraft can do it in one trip, but the fighters cannot. Send them together, and they can top up when needed. Now they can cross the Atlantic in one go without needing to stop for fuel.

In addition to the other suggestions, possibly look at Operation Chrome Dome. Sometimes you need to have planes constantly airborne, and in-air refueling significantly reduces the number of planes (and to some extent bases) required to do that.

Which bit of "… extend the range how exactly?" doesn't work for you?
You give the plane more fuel, it flies further…

See also:  https://en.wikipedia.org/wiki/Operation_Black_Buck

You may want to watch this.

Tankers have a much bigger range than fighter jets and can carry far more fuel, and can also work in tandem. See Operation Black Buck for an exteme example of using air to air refueling to increase your operational capability.
Tankers also let other aircraft loiter in the area longer than they could, otherwise. See, for example, tankers being launched as part of domestic quick reaction operations in the UK.
There's also other extreme examples such as the SR71 which didn't take off with its full payload of fuel to reduce stress on the brakes and tires during takeoff and also ensure it could successfully take off should one engine fail. Even modern fighters may not be able to be fully fueled in some configurations and even bombers may be range limited in the same way.
From reading your post, I think your primary fallacy is the assumption tankers must be available in R. This isn't true at all, the tankers might well be based with the aircraft they serve or nearby. They just have far larger fuel stores. R doesn't have to be a base, or even exist in literal terms - plenty of refueling simply happens en route, not as a diversion.
It should be said that air to air refueling is a well planned, and well orchestrated concept and can be something of a dance with tankers refueling tankers, and switching onto station etc. It's not as simple as just having a tankers floating about.

Didn't some versions of the Harrier also have to take off without a full fuel load if fully armed and operating in VTOL mode?

For example, the KC-10 tanker has a loaded range of 3800nmi, while the F-18 has a loaded range of 1200nmi and a combat range of 400nmi.  The KC-10 carries about 36 times the amount of fuel that an F-18 can.

It might be worth adding a point of why the fighters don't just have larger fuel tanks.

A bomber's mission is bombing and it carries bombs. A tanker's mission is fueling and it carries fuel.

For a real-life example of this, "point R" for the evacuation of Kabul was out over the Gulf of Oman.

@hobbs yes and no (buddy refuelling exists)

Are there some kind of safety reasons why a tanker can't refuel itself?  I mean, it's got excess fuel on board, and just needs some way to transfer some of that to its own tanks like reserves.

@Michael on at least some tankers its simply the same fuel system, the fuel is shared. I would imagine that's pretty common, but don't quite me on it!

Ah... if there were a case where some of the aircraft took different variants of fuel (no idea if this would ever be the case) then possibly the tanker could only refuel the fighters but itself would have to be fueled by another tanker or something like that.

@Michael It's pretty much all the same (save for extreme exceptions like the U2), there's even a NATO standard for it too to ensure fuel availability and cross compatibility on operations.

Correct answer: the tanker go alongside the fighter, at some point refuel it and goes back while the fighter goeson

Tankers do not need to be based at point R... they can and often travel quite far to refuel other airplanes.
Why not base tankers at point R?

They are considered high value airborne assets (like AWACS).  Too valuable to lose this capability to attack.
Unlike tactical aircraft, they often lack defensive systems and require combat air patrols to keep them safe.
Tankers can use or offload the fuel they carry.  Some can also receive fuels... giving them near unlimited range to travel to the point of refueling.

Why is air refueling a strategic capability?

Force Extension (Airbridge): This gives other airplanes near unlimited range.  B-2 bombers only have one base in the US Midwest... but can hold any target on the planet at risk by staging multiple refuelings along the way.

Force Multiplication:  This give war planners more capability than is actually deployed to support the mission.  With air refueling support, fighters can loiter longer over targets and carry increased weapons loads.

Anyway tankers are more like (and may be derived from) airliners - designed to fly laden between continents, taking a good few hours to do so, so even if they're not set up to use the fuel they carry for others, they've still got a lot of range.  In brief tankers make point R

Well said.  Another way to state it... NKAWTG.

Two simple examples:
First, say you want to get fighters from the US to Europe.  The AR divert fields might be Bangor, Maine, then St John's Newfoundland, then Keflavik, Iceland, then someplace in Ireland.  You don't have to fly them point-to-point from each of these getting gas on the ground along the way.  You don't even have to overfly any of them.  You simply plan your route so that there is enough gas in the tanks of each fighter so that if "this" AR fails, they have the gas to turn north to whatever divert field is closest "now".  So they get one long flight to Germany, rather than several shorter ones, which would add up to more time & more opportunities for an aircraft to break.
Second, let's say you have multiple bases in one Saudi Arabia, and fighters will go attack targets in Iraq -- a Desert Storm type of scenario.  The fighters take off from bases fairly far south in Saudi.  Shortly before going into hostile airspace, they tank up so their tanks are full.  Their fuel plan is to conduct the mission in hostile territory, and tank again after returning to Saudi airspace.  If that AR fails, then they divert to the farthest north (closest) friendly base; in the 99% case that the post-strike AR goes fine, then they can return to their actual base farther away.  They didn't have the fuel at takeoff to fly all those miles, but because of the tanker, the mission works fine that way.
Having to defend the AR divert fields, or not (simple Atlantic crossing), isn't really related to the refueling itself.
Also, the tankers, having all kinds of capacity for fuel, can be based anywhere -- not at any of the AR divert bases.  They may accompany the receiver all the way across the pond (common when dragging fighters across the Atlantic), or they may orbit in friendly airspace (a Desert Storm scenario).  But they probably are NOT based at the far-north base that'd be the divert field for a fighter that couldn't AR.  With their range, they can be based pretty far away just fine.

Air to Air refueling allows a jet to get to somewhere beyond its actual range on internal gas.
If a jet has a range when loaded of 800 miles, and the target is 1000 miles away, that jet cannot do the full round trip on a single tank of gas.
For Navy jets off a carrier, often they cannot take off with both a full tank of gas AND a full load of munitions.
So, take off with 1/2 gas, and fill up in the air.
Fly to the target, drop munitions, return...tanking on the way back.
Air to air refueling is a range extender.
"Ok cool, so why doesn't your enemy attacks R, which is unprotected"
(R being the refueling aircraft)
Generally, it IS protected, or out of range from the attackers.
Even just a commonplace issue of flying an F-16 from Shaw AFB, SC to Aviano AB Italy...you'll need to tank.
A KC-10 from McGuire AFB NJ, orbiting over the Atlantic, to fill up on the way.

Another point: political sensibility
Imagine the US conducting military operations in a region with no strong allies in the neighbourhood. Why would a nation, which is on barely friendly terms with the US allow armed US fighters to land and refuel? This is unlikely to happen.
However, there's always the option to place a tanker over international water to do the refueling.
Air-refueling allow you to conduct operations independent of land bases, which most often belong to other nations, which in turn are at best moderately friendly.

There can also be allies which for political reasons would balk at hosting nuclear-capable warplanes, or being the launch point for an attack, but which might agree to host logistics and supply equipment, including tankers, or at least allow refueling in their airspace. Political compromises can result in some pretty random combinations of restrictions.

Another reason could be if you're conducting a high sensitivity operation, such as inserting US Special Forces to aid friendly guerrilla forces in denied territory. The insertion aircraft (A USAF MC-130 Combat Talon, for example) would be launched from a secure base in the US and air refuel along the way, without having to stop at allied bases along the way, which would introduce or increase the risk of compromise.


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What is this thin strake on the side of the Eurofighter Typhoon?

It looks like it's right where the canard would be if it was close-coupled.

EDIT: I found a nice picture!

Strakes are used to "correct" and/or "channel" the airflow under specific conditions.
That one in particular on the Eurofighter looks like a strake producing a nice vortex at high AoA which rolls over the wing streamlining the airflow and helping in generating lift.
Indeed this photo nicely depicts and confirms that:

Bonus material
Strakes can also be see for example on the F-18:

In this particular case, they channel the airflow at high AoA in order to avoid that the turbolent airflow detaching from the trailing edge bumps into the the vertical stabiliser giving some buffeting.